Today is the summer solstice in the Northern Hemisphere, and the winter solstice in the Southern. For those in the north, today will be the longest day of the year and tonight will be the shortest night.
Although you would think that the Earth would be closest to the Sun during the summer, actually we're about 3 million miles farther away than we are in winter. But our planet is tilted on its axis, and at this time of year, the Northern Hemisphere is tilted toward the Sun, receiving more direct radiation for longer periods of time each day.

It is that slight tilt, only 23.5 degrees, that makes the difference between winter and summer.

We consider the summer solstice to be the first official day of summer, but in the ancient world, it was celebrated as Midsummer, and it was thought to be a time when plants had particularly magical properties.

Fairies, ghosts, and spirits were thought to be especially active too, and Shakespeare's play A Midsummer Night's Dream reflects a lot of those traditional beliefs.
In modern times, Midsummer's Eve is celebrated sometime between June 21 and June 24; it's still a major holiday in Scandinavia, Latvia, and other locations in Northern Europe, second only to Christmas.
It dates back to pre-Christian times, and people take a three-day weekend to dance around maypoles, clean and fill their houses with fresh flowers, and burn straw witches in bonfires to remember the witch burnings of the 16th and 17th centuries.

One of the biggest destinations for the summer solstice is Stonehenge, on England's Salisbury Plain; it's the only day of the year the park service offers...the opportunity to stay at the monument overnight.
(Writers Almanac)